Conventional multi-axle assemblies for trucks or trailers may comprise two or more axles. In the case of a two axle arrangement, four leaf springs are used. Two leaf springs are on each side of the frame. The remote ends of each pair of springs are usually supported within hanger brackets which are secured to the frame. The adjacent ends of each pair of springs are supported by a pivotally mounted equalizer. The equalizers are in turn supported by hanger brackets which are secured to the frame.
When the front wheels in the tandem suspension, for example, encounter a condition in the road producing a vertical movement of the front axle, the resultant deflection in the front springs is transmitted in part by the equalizer to the rear springs. This results in an equalizing effect in which any vertical displacement of either axle is distributed between the springs which minimizes the weight differential between the axles.
In the instance of a vehicle equipped with a prior art equalization tandem one problem concerns inadequate and abrupt load equalization from one axle to another. As one end of the equalizer rotates upwardly at the point at which the spring end contacts the leaf spring, the other end abruptly rotates downward. This action causes abrupt and unequal moments about the center pivot point of the equalizer; inequality increases as the range of equalizer movement increases.
The end results of improper load equalization are numerous. If the axle which encounters bumps and overloads is unpowered, a loss of traction can occur on the powered axle. The suspension components, and springs in particular, are subjected to higher stresses and therefore their service life is shortened appreciably. As the leaf springs are subjected to overload they progressively lose capacity to absorb energy and therefore transmit more energy through the mounting brackets to the vehicle frame. Increased input of energy into the vehicle frame can often contribute at given loads, speeds and highway conditions to damage to the vehicle. Further, wherever an unequal load distribution occurs within a tandem suspension, the axle which is overloaded transmits its load to the roadway in a manner that can be detrimental to the roadway. The foregoing arguments also apply to suspensions comprising more than two axles.
Representative of the art is U.S. Pat. No. 4,033,606 to Ward (1977) which discloses an equalized suspension system is provided for the truck of a truck-trailer combination of the tandem axle type. The suspension on each side of the vehicle includes independent semi-elliptic leaf springs mounted in tandem relationship one behind the other, and which are respectively connected at their midpoint to the forward and rearward axles. The remote ends of each spring are supported in rolling contact with respective roller assemblies housed by respective brackets which, in turn, are secured to the frame of the truck. An equalizer support bracket is provided between the adjacent ends of each pair of springs and an equalizer is pivotally supported within each equalizer support bracket. The adjacent ends of the springs are pivotally attached to shackles which, in turn, are pivotally mounted to the respective equalizers.
What is needed is a multi-axle leaf spring suspension system having a compliant equalizer. The present invention meets this need.